The present invention relates to a dielectric filter. More specifically, it relates to a dielectric filter having a plurality of resonators.
Personal radio telephones, mobilephones and similar devices employing microwaves include compact dielectric filters which demonstrate high selectivity. The following conventional dielectric filters relates to the present invention.
(1) A dielectric filter as disclosed in Japanese Pat. Laying-Open No. 292401/1986 is composed of a pair of resonators connected with each other. Each of the resonators comprises a dielectric block made of ceramic material having a through-hole, an inner conductive layer coating the inner surface of the through-hole, an outer conductive layer coating the outer surface of the dielectric block, and a short-circuiting layer for short-circuiting the inner and outer conductive layers. The resonators are connected by, for example, soldering the outer conductive layers. Between the facing surfaces of the adjacent resonator is a slit formed by removing a part of the outer conductive layers of the resonators.
In a dielectric filter, a passband can be obtained usually by controlling the degree of coupling between the adjacent resonators. That is, the passband is controlled by altering the size of the area of layer removed between the resonators in order to change the degree of coupling between the resonators. For example, as the layer-removed area is enlarged, increasing the degree of coupling, the passband is thereby widened.
This dielectric filter has the following drawbacks. The area of the conductive layers on the connecting surfaces of the resonators is necessarily reduced, as the layer-removed area is enlarged. This results in a reduction in the making areas available for soldering, whereby mechanical connection between the resonators is weakened. Weakened connection causes instability in the passband characteristics of the filter. When a slot is obtained by masking a part of the dielectric blocks during the process of coating the blocks with outer conductive layers, inaccurate positioning during the masking process or the coating process may alter the degree of coupling.
(2) Another dielectric filter as disclosed in Japanese Pat. Laying-Open No. 24702/1988 comprises a filter body and a frame for containing the filter body.
The filter body includes a dielectric block having a pair of through-holes, inner conductive layers coating the surface of the through-holes, an outer conductive layer coating the outer surface of the dielectric block, and a short-circuiting layer for short-circuiting the inner conductive layers to the outer conductive layer. Provided between the through-holes in a coupling hole extending in parallel to the through-holes. Thus, the filter body constitutes a filtering circuit formed by the coupling of a pair of resonators, whereby an electromagnetic coupling is generated.
The frame has a pair of metal plates disposed in parallel, and a supporting plate laterally extending between the bottom portions of the metal plates. The metal plates have legs for earthing at the bottom ends of sides.
In third dielectric filter, the filter body is contained in the frame so that the through-holes face the supporting plate.
This dielectric filter has the following drawbacks:
(a) Making the frame is difficult. Since the frame has a complex shape as described above, producing the frame is necessarily a complex process.
Furthermore, assembling the filter body to the frame is necessarily a complex process. Specifically, the filter body must be first inserted into the frame, and secondly, the filter body must be soldered to the frame; consequently the assembly is necessarily complex.
Accordingly, the above dielectric filter is necessarily expensive due to its inferior productibility.
(b) The opening of the coupling hole in the filter body is not covered by the frame, so that the dielectric filter is likely to pick up outside noise. Additionally, after the dielectric filter has been mounted on a circuit board, noise emitted from the coupling hole may deteriorate other electronic devices near the dielectric filter.